1. Field of the Invention
The invention relates to methods for the fractionation of polymeric mixtures and more particularly relates to methods of separating the various polymer components of polymerized fatty acids.
2. Brief Description of the Prior Art
Polymeric fatty acids are prepared by the polymerization of fatty acids, using techniques well known to those skilled in the art; see for example the polymerization described in U.S. Pat. No. 3,157,681. These polymeric fatty acids are sometimes referred to in the art as "dimer fatty acids" because the predominant constituent is the dimer of the fatty acid. However, the polymerized fatty acids are complex mixtures, containing unreacted monomeric compounds, trimer forms and higher polymeric forms of the fatty acids.
Commercially available polymeric fatty acids prepared from tall oil fatty acids are representative of polymeric fatty acids and may have a composition as follows:
______________________________________ % BY WEIGHT ______________________________________ C.sub.18 monobasic acids (monomer) 0-5 C.sub.36 dibasic acids (dimer) 60-95 C.sub.54 and higher polybasic acids (trimer) 1-35 ______________________________________
The relative ratios of monomer, dimer and trimer in unfractionated polymeric fatty acids are dependent on the nature of the starting material and the conditions of polymerization.
Heretofore, fractionation of the polymeric fatty acids to separate the monomer and dimer-trimer components has been done by conventional distillation and molecular distillation, and the separation of dimer and trimer components only by molecular distillation. These techniques are of course costly procedures and are adversely impacted upon by any ash in the dimerization product.
The method of the present invention provides a relatively inexpensive means to fractionate polymeric fatty acids.
The employment of solvent gases as liquids in their near critical state and as fluids in the supercritical state as extracting solvents has been previously described; see for example Francis, Physical Chem, 58,1099 (1954) and Ind. Eng. Chem. 47, 230 (1955). Near critical and supercritical fluids, including hydrocarbon gases have been suggested as solvents for a wide range of materials; see for example U.S. Pat. No. 3,969,196. Despite the fact that the solvation properties of gases in their near critical and supercritical states and especially of hydrocarbon gases, has been known, the application of this knowledge has not been applied to the fractionation of polymeric fatty acids.